This invention relates to optical data transmitters. In particular the invention relates to optical data transmitters that have relatively high tolerance to effects of fiber dispersion and nonlinearity compared with conventional NRZ fiber-optic transmitters.
In the information age, the demand for data networks of higher and higher data capacities, at lower and lower costs is constantly increasing. This demand is fueled by many different factors, such as the tremendous growth of the Internet and the World Wide Web. The increasing numbers of on-line users of the Internet and the World Wide Web have greatly increased the demand for bandwidth because of the proliferation of bandwidth-intensive applications such as audio and video streaming and file transfer.
Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. Optical fiber offers much higher bandwidths than copper cables and is much less susceptible to various types of electromagnetic interference and other undesirable effects. As a result, it is the preferred medium for transmission of data at high data rates and over long distances.
At very high data rates, chromatic dispersion in optical fiber transmission lines causes waveform deterioration and becomes a limiting factor in standard single-mode optical fiber. Although dispersion-shifted optical fiber exists, which exhibits very low dispersion at optical fiber transmission wavelengths, there is a large installed base of standard signal-mode optical fiber. Thus, there is a great demand for dispersion tolerant data transmission systems.
Correlative coding techniques can be used to enhance tolerance to fiber dispersion and other non-linear effects. Correlative coding techniques, also known as partial response signaling, were developed in the 1960s. One type of correlative coding technique is called duobinary signaling. Duobinary coding was first published in 1963 by A. Lender in xe2x80x9cDuobinary Technique for High Speed Data Transmission,xe2x80x9d IEEE. Trans. Commun. Electron., vol. CE-82, pp. 214-218, May 1963.
A duobinary (DB) signal is created by delaying a binary bit sequence by one full bit and then adding the delayed binary bit sequence to the original bit sequence. See, for example, U.S. Pat. No. 5,917,638 issued to Franck et al. The DB signal can be expressed as follows:
DBi=mi+mixe2x88x921.xe2x80x83xe2x80x83(1)
The DB signal is a three level sequence with one half of the bandwidth of the binary bit sequence m. Duobinary coding reduces the signal bandwidth by mapping a binary data signal having two levels to be transmitted into a three-level signal having three meaningful values or levels. See, for example, U.S. Pat. No. 5,867,534 issued to Price et al. The signal received by the receiver is interpreted in terms of three levels rather than two levels. The reduction in signal bandwidth reduces the waveform deterioration caused by chromatic dispersion.
Duobinary coding has been implemented with optical signals using a Mach-Zehnder interferometric modulator biased at the quadrature point and a three level intensity detector as the receiver. See for example, X. Gu and L.C. Blank, xe2x80x9c10 GB/s unrepeatered three-level optical transmission over 100 km of standard fibre,xe2x80x9d Electronics Letters Vol. 29 No. 25 pp 2209-2210 (received Oct. 8, 1993).
An optical duobinary transmission system has been proposed that uses a two-level (on, off) approach. See, for example, K. Yonenaga, S. Kuwano, S. Norimatsu and N. Shibata, xe2x80x9cOptical duobinary transmission system with no receiver sensitivity degradation,xe2x80x9d Electronics Letters Vol. 31 No. 4 pp 302-304 (received Dec. 7, 1994). Since typical optical detectors respond to optical intensity as opposed to amplitude, decoding is automatically achieved at the detector and duobinary decoding is not necessary. The system requires that the phase of the xe2x80x9conxe2x80x9d state signal take the values of either xe2x80x980xe2x80x99 or xe2x80x98xcfx80xe2x80x99. The two xe2x80x98onxe2x80x99 states correspond to the xe2x80x98+1xe2x80x99 and xe2x80x98xe2x88x921xe2x80x99 states of the duobinary signal, and the xe2x80x98offxe2x80x99 state corresponds to the xe2x80x980xe2x80x99 state of the duobinary signal.
The optical duobinary signal is generated by driving a dual-drive Mach-Zehnder modulator with push-pull operation. Two duobinary signals for driving the Mach-Zehnder are generated from original binary signals by using two duobinary encoders. The two duobinary signals are applied to two electrodes of the Mach-Zehnder modulator. The xe2x80x980xe2x80x99 state of the duobinary signal is equal to the zero level. The xe2x80x98+1xe2x80x99 and xe2x80x98xe2x88x921xe2x80x99 states have the same magnitude and opposite signs for push-pull operation.
The dispersion tolerant optical data transmitter of the present invention performs preceding. The precoding can be accomplished either at the line rate or at a lower rate if a multiplexer is used. Decoding is performed at the receiver by a square law detector. In one embodiment, a delay of less than a full bit period is used.
The dispersion tolerant optical data transmitter of the present invention is approximately a factor of four less sensitive to chromatic dispersion than conventional optical transmitters. Also, the dispersion tolerant optical data transmitter of the present invention is less sensitive to fiber nonlinearities and can transmit at higher power levels, and therefore, longer distances, because the carrier is suppressed.
Accordingly, in one aspect, the present invention is embodied in an optical data transmitter including a precoder that converts an input data signal to a binary precoded data signal and to a complementary binary precoded data signal at an output and a complementary output, respectively. In one embodiment, the precoder is a serial precoder.
In another embodiment, the precoder is a parallel precoder having n sets of parallel data inputs that receive n sets of parallel data. The parallel precoder generates n sets of parallel precoded data at n sets of parallel outputs from the n sets of parallel data. The parallel precoder also includes a multiplexer having n sets of parallel data inputs that are coupled to the n sets of parallel outputs of the parallel precoder, respectively. The multiplexer generates the binary precoded data signal and the complementary binary precoded data signal at the output and the complementary output, respectively.
The optical data transmitter also includes a delay element coupled to one of the output and the complementary output of the precoder. The delay element delays one of the complementary binary precoded data signal and the binary precoded data signal relative to the other at an output of the delay element, by a time corresponding to less than one bit period of the binary precoded data signal.
In one embodiment, the delay element delays one of the complementary binary precoded data signal and the binary precoded data signal relative to the other by a time corresponding to between 0.4 and 0.9 of the bit period of the binary precoded data signal. In one embodiment, the delay element includes a variable delay element. In one embodiment, the delay element is selected to increase dispersion tolerance of a communication system that includes the optical data transmitter.
The optical data transmitter further includes a differential amplifier having a first input that is coupled to an output of the delay element and a second input that is coupled to one of the output and the complementary output of the precoder. The differential amplifier converts the binary precoded data signal and the complementary binary precoded data signal to a four-level data signal and to a complementary four-level data signal at a differential output and a complementary differential output, respectively.
In one embodiment, the four-level data signal includes a minimum amplitude, a first intermediate amplitude, a second intermediate amplitude, and a maximum amplitude. An average of the minimum amplitude and the maximum amplitude is substantially equal to an average of the first intermediate amplitude and the second intermediate amplitude.
The optical data transmitter also includes an optical data modulator having a data input that is coupled to one of the differential output and the complementary differential output of the differential amplifier. The optical data modulator modulates an amplitude of the optical signal applied to an optical input of the optical data modulator in response to at least one of the four-level data signal and the complementary four-level data signal, respectively, to generate a modulated optical output signal.
In one embodiment, the optical data modulator includes a single input zero-chirp Mach-Zehnder modulator. In another embodiment, the optical data modulator includes a second data input that is coupled to the other one of the differential output and the complementary differential output. The optical data modulator modulates an amplitude of the optical input signal in response to the four-level data signal and the complementary four-level data signal to generate a modulated optical output signal. In yet another embodiment, the optical data modulator includes a differential input Mach-Zehnder modulator.
In one embodiment, the modulator includes a predetermined operating point that is chosen so an intensity of the modulated output optical signal is minimized when the amplitude of the four-level data signal is substantially equal to an average of the four levels of the four-level data signal. In one embodiment, the modulator includes a predetermined operating point that is chosen so an intensity of the output optical signal is minimized when the amplitude of the four-level data signal is substantially equal to an average of the four levels of the four-level data signal, and the amplitude of the complementary four-level data signal is substantially equal to an average of the four levels of the complementary four-level data signal.
In one embodiment, the optical data transmitter also includes a bias voltage source that adjusts an average amplitude of at least one of the four-level data signal and the complementary four-level data signal to change an operating point of the optical data modulator.
In one embodiment, the optical data transmitter further includes a filter having an input that is coupled to one of the differential output and the complementary differential output of the differential amplifier and having an output that is coupled to the data input of the optical data modulator. The filter reduces the bandwidth of at least one of the four-level and complementary four-level data signal. In one embodiment, the filter provides an adjustable cut-off frequency.
In one embodiment, the optical data transmitter further includes at least one of a first and a second filter. The first filter includes an input that is coupled to the differential output and an output that is coupled to the first data input of the modulator. The second filter includes an input that is coupled to the complementary differential output of the differential amplifier and an output that is coupled to the second data input of the optical data modulator. The first and the second filters reduce the bandwidth of the four-level and the complementary four level data signal, respectively. In one embodiment, at least one of the first and the second filter provides an adjustable cut-off frequency.
In another aspect, the present invention is embodied in a method for coding an optical data signal. The method includes converting an input data signal to a binary precoded data signal and to a complementary binary precoded data signal. In one embodiment, converting the input data signal to a binary precoded data signal and to a complementary binary precoded data signal includes converting n sets of parallel data signals to n sets of parallel precoded data signals, and multiplexing the n sets of parallel precoded data signals to generate the binary precoded data signal and the complementary binary precoded data signal.
The method also includes generating a delayed data signal by delaying one of the complementary binary precoded data signal and the binary precoded data signal relative to the other by less than one bit period of the binary precoded data signal. In one embodiment, the generating the delayed data signal includes delaying one of the complementary binary precoded data signal and the binary precoded data signal relative to the other by a time corresponding to between 0.4 and 0.9 of the bit period of the binary precoded data signal.
In another embodiment, the generating the delayed data signal includes delaying by a time that increases dispersion tolerance of a communication system using the method for coding an optical data signal.
The method further includes converting the delayed data signal and the other of the complementary binary precoded data signal and the binary precoded data signal to a four-level data signal and to a complementary four-level data signal. In another embodiment, the four-level data signal includes a minimum amplitude, a first intermediate amplitude, a second intermediate amplitude, and a maximum amplitude. An average of the minimum amplitude and the maximum amplitude is substantially equal to an average of the first intermediate amplitude and the second intermediate amplitude.
The method further includes modulating an optical signal with at least one of the four level data signal and the complementary four-level data signal, thereby generating a modulated optical output signal with four amplitude levels.
In one embodiment, the method further includes adjusting at least one of the first intermediate and the second intermediate amplitude relative to the minimum amplitude and to the maximum amplitude to increase dispersion tolerance of a communication system using the method for coding an optical data signal. In another embodiment, adjusting at least one of the first intermediate and the second intermediate amplitude relative to the minimum amplitude and to the maximum amplitude includes filtering at least one of the four-level data signal and the complementary four-level data signal.
In one embodiment, the adjusting at least one of the first intermediate amplitude and the second intermediate amplitude relative to the minimum amplitude and to the maximum amplitude includes delaying one of the complementary binary precoded data signal and the binary precoded data signal relative to the other by less than one bit period of the binary precoded data signal.
In one embodiment, an amplitude of the modulated optical output signal is substantially zero when the at least one of the four-level data signal and the complementary four-level data signal is substantially equal to an average of amplitudes of the four levels comprising the at least one of the four-level data signal and the complementary four-level data signal.